The technique consisting of laterally drilling from a previously drilled main well is not a new form of technology. This main well may be an open hole well, that is to say uncased, or cased by a casing string. In the first case, the well generally has to be plugged at the depth at which point the lateral drilling is to be initiated. This may be achieved by setting a cement plug which will provide the support necessary for a directional drill string to begin lateral drilling. This drill string is conventionally equipped with a downhole motor and a deflection tool, such as a bent sub. It is also possible to perform a rotary drilling operation by using a deflection device commonly called a "whipstock", which is fastened to, or fixed in place of the plug. This latter technique, which is quite old, is more difficult to master in open holes where it is difficult to correctly position the whipstock in the well. If the main well is already cased, the technique, which is identical, imposes an additional operation involving the milling of a window in the casing in order to utilize a directional drill string through the opening provided thereby. This operation requires a milling tool adapted to the material of the casing in which a window is to be cut out.
The object of these procedures, known as "side track" operations, is generally to abandon the lower part of the main well located at a lower level than said the plug or "whipstock". In this case, the completion of the new Well will be conventional, that is to say, the casing is either run to surface or is suspended in an existing string through well-known means, for example by use of the hanging device commonly known as a "liner hanger".
U.S. Pat. No. 4,807,704 mentions a known system and method for completing several laterals from a main well, but the equipment of the main and of the lateral well is complex and restricts the inner space of the main well, making any access to the lower part of the main well impossible. Moreover, drilling the lateral well requires a stage of milling in the casing string of the main well.
U.S. Pat. No. 4,852,666 mentions a known device and method for drilling lateral wells with respect to a horizontal drain. However, this document does not disclose a technique allowing lateral wells to be drilled from a main well which is already cased. Besides, it does not allow the lateral well to be completed with a casing.